The present invention relates to refrigeration systems and particularly such systems as employed on board a motor vehicle for providing cooling of a compartment such as the passenger compartment for the vehicle. Currently automotive vehicle passenger compartment air conditioning systems typically employ an engine driven compressor for circulating refrigerant through an exothermic heat exchanger or condenser to an expander for dropping the pressure of the refrigerant for circulation through an endothermic heat exchanger or evaporator located for cooling a flow of air thereover directed to the passenger compartment. The refrigerant is returned to the compressor inlet from the evaporator for recirculation. Typically the compressor is operated by an electrically energized clutch for connecting the compressor to the engine.
Heretofore, such automotive air conditioning systems have proven generally effective; however, the means of controlling the flow of refrigerant to the evaporator for cooling the passenger compartment in the face of varying ambient thermal loading has been accomplished either by utilizing a temperature sensor on the evaporator and using the sensed temperature to control compressor clutch cycling, or by utilizing a pressure sensor in the refrigerant line disposed between the evaporator outlet and the compressor inlet with the pressure switch controlling the compressor clutch. Alternatively, it is known to utilize a temperature sensor in the refrigerant line at the evaporator inlet.
When an automotive A/C system is working under certain conditions, such as higher cooling capacity output (high vehicle speed and engine RPM ), and low cooling load (low blower speed @ recirculation mode or low ambient temperature @ outside air mode), and higher relative humidity of air, the condensate separated from the moist air will freeze on the fins of the evaporator if the condensate temperature reaches 0.degree. C. (32.degree. F.) or below. The frozen condensate will then block the air stream passing through the evaporator, reduce heat transfer effectiveness between the cold refrigerant and the hot air, and will cause the refrigerant system to malfunction, and eventually cause discomfort in the passenger compartment.
In order to prevent condensate freezing on the evaporator and to maintain the normal operation of the air conditioning system, one of the following three different control means has been utilized, namely a system having 1.) a temperature sensor installed in the refrigerant line (cold control); 2.) a pressure transducer installed in the refrigerant line (pressure control); and 3.) a temperature sensor inserted between the fin arrays in the air side of the evaporator (fin sensor control). Each of these system arrangements requires a corresponding control strategy and algorithm to (a) use the temperature or pressure outputs obtained from the temperature sensors or pressure transducer mentioned above as the control parameter(s); (b) use the background information obtained from other subsystems/components ( e.g., blower speed selection, air quality door position, ambient conditions, clutch cycling status, etc. ) through on-board real-time communications; (c) define a set point or operating zone based upon the real-time background information; (d) compare the real-time data input(s) with the defined set point or operating zone and calculate the error(s) between the set point or operating zone and the real-time data input(s); and (e) make control decision and command the desired control action(s).
In systems employing a fin temperature sensor that is inserted among the fin arrays on the evaporator surface, problems have been encountered; namely: 1.) it is very difficult, if not impossible, to define a meaningful sensor location on the evaporator surface. Theoretically, the coldest spot on the surface should be chosen as the sensor location to prevent the evaporator from freeze up. In reality, however, the coldest spot moves randomly around the evaporator surface dependent upon the operating and ambient conditions, as indicated by testing. This uncertainty has resulted in low quality of data inputs, poor accuracy of control, and longer calibration time, and 2.) the complexity of interfacing the sensor with other components has caused difficulty in packaging the system.
Problems have also been encountered in a system employing a pressure sensor; namely: 1.) the occurrence of rapid pressure changes occurring during the compressor clutch cycling which creates less temperature accuracy, whereas, the purpose of the freeze up prevention is to control temperature. There is a one-to-one relationship between the pressure value and the temperature value when the refrigeration system operates at steady-state condition. This one-to-one relationship does not exist when the refrigeration system operates in a dynamic mode; and, thus it is extremely difficult to control temperature based upon the pressure input; 2.) it is relatively costly, and, it has more parts and interfaces which makes packaging more difficult.
In the operation of the above described system, the cabin air temperature control is accomplished by controlling the refrigerant flow entering the evaporator through cycling the clutch of compressor. When the clutch is engaged, the compressor pumps refrigerant through evaporator which provides cooling to the blower air stream. When the compressor clutch is disengaged, the compressor stops and no refrigerant flows through evaporator. In actual vehicle operation, the cycling frequency ranges normally from 0 to about 6 cycles/min. The dynamic on/off cycling rate of the clutch has a substantial impact on the stability of cabin air temperature control. In this regard, the control algorithm developed should improve or optimize refrigerant system performance through balancing the needs of evaporator freeze up prevention and cabin air temperature quality (stability).
In order for the control algorithm fulfill its task providing accurate and optimum temperature control an accurate sensor must be provided and properly located. The control strategy and algorithm which will be used to control the clutch cycling operation based on the sensor signal are thus critical.
It has been thus desired to provide a sensor arrangement and algorithm for an automotive air conditioning system to operate the compressor clutch in accordance with a systematic control strategy that will maintain optimum compressor operation for preventing the formation of ice on the evaporator and yet provide the desired cooling and comfort level for a passenger compartment irrespective of the ambient conditions experienced during vehicle operation or the particular blower speed settings and air flow operation selected by the vehicle operator. It has further been desired to provide such a control sensor and algorithm integration for an automotive air conditioning system which is low in cost and easy to install during vehicle manufacturing.